This study is a multi-center, open-label dose escalation Phase I trial designed to assess the safety, pharmacokinetics and preliminary activity of BMS-188667 administered as four intravenous injections in patients with psoriasis. BMs-188667 (CTLA4Ig) represents a new class of agent that targets the blockade of the second signal in T cell activation. There is increasing evidence that the T cell is pivotal in the pathogenesis of psoriasis. The drug has the potential to provide another treatment alternative for patients with severe disease. The trial may also help elucidate the immunologic mechanisms involved in the pathogenesis of psoriasis.